(i) Field of the Invention
The present invention relates to a method for racemization of optically active serine. More particularly, the present invention relates to a method for racemization of serine by treating serine with an alkaline aqueous solution in the presence of pyridoxal phosphate or salicylaldehyde, and an alkaline compound.
Serine is used in the preparation of cosmetics, as a constituent of solutions for infusion and as a raw material in the preparation of antibiotics. It is also an amino acid which can be used as a raw material for the synthesis of L-tryptophan, L-tyrosine and L-cysteine. According to the specific end use, a particular isomer of serine; namely, the L-isomer, the D isomer or the racemic mixture is used. Serine obtained by proteolysis, fermentation and by enzymatic reaction is usually the L-isomer. Serine obtained by an organic synthesis is usually the racemic mixture. The demand for the L isomer, D isomer or the racemic mixture is not always equal. Therefore, it is convenient if optically active serine can be racemized, optionally in combination with optical resolution, to obtain the desired optically active isomer or so that racemic mixture can be prepared from the L-isomer or the D-isomer or their racemic mixture.
(ii) Description of the Prior Art
As methods for racemization of serine, several methods are known which are as described as follows:
(1) The method of heating optically active amino acid at a temperature of above 150.degree. C. in a closed vessel (U.S. Pat. No. 3,213,106).
(2) The method of heating optically active amino acid at a temperature of between 105.degree. to 150.degree. C. and at a pH value of about 5 to 8 with m-xylene-4-sulfonic acid, ammonia and water (Japanese published Pat. No. (76) 51-41616).
(3) The method of racemization of amino acid in the presence of lower fatty acid and aliphatic or aromatic aldehyde (Japanese Laid Open Patent No. 57-12150, J. Org. Chem. 48, 843-846 (1983)).
(4) The method of racemizing serine which is conducted in the presence of cultured microorganisms, cultured strains or the extracts thereof (Japanese published Patent No. 58-52637, 58-52638, 61-4518).
(5) The method of heating optically active amino acid in the presence of strong acid or strong alkaline (Advances in Protein Chemistry 4, 339 (1948)).
(6) The method of heating optically active amino acid in the presence of pyridoxal (or pyridoxal like compound) and a metallic ion such as aluminum, iron, or copper (J. Biol. Chem, 199, 669 (1952), Bull. Chem. Soc. Jpn., 35, 1422 (1962)).
When these known methods for the racemization of optically active amino acids are applied to serine, several disadvantages have been found. More specifically, according to the methods (1), (5) or (6), the decomposition of serine dominates the racemization and as a result, the yield of DL-serine decreases. In method (2), it is necessary to use a large amount of expensive m-xylene-4-sulfonic acid as well as heating to a high temperature in the range of 105.degree. to 150.degree. C. In method (3), the racemization has to be done in the presence of a high concentration acetic acid and, in addition, the decomposition of the serine itself occurs. In method (4), the preparation of the microorganisms takes a long time, and in case of the existence of substances which inhibit the enzymatic reaction, this method is not applicable.
Thus, several methods for the racemization of optically active amino acid have been known, but commercial implementation of acceptable methods of achieving racemization has been difficult.